Author

Degree

Doctor of Philosophy

Program

Microbiology and Immunology

Supervisor(s)

Dr. John McCormick

Abstract

Superantigens (SAgs) are microbial toxins that cross-link T cell receptors with major histocompatibility complex (MHC) class II (MHC II) molecules leading to the activation of large numbers of T cells. Herein, the development and preclinical testing of a novel tumour-targeted SAg (TTS) therapeutic built using the streptococcal pyrogenic exotoxin C (SpeC) SAg and targeting cancer cells expressing the 5T4 tumour-associated antigen (TAA) was described. To inhibit potentially harmful widespread immune cell activation, a SpeC mutation within the high-affinity MHC II binding interface was generated (SpeCD203A) that demonstrated a pronounced reduction in mitogenic activity, yet this mutant could still induce immune cell-mediated cancer cell death in vitro. To target 5T4+cancer cells, a humanized single-chain variable fragment (scFv) antibody to recognize 5T4 (scFv5T4) was engineered. Specific targeting of scFv5T4 was verified. SpeCD203A used to scFv5T4 maintained the ability to activate and induce immune cell-mediated cytotoxicity of colon cancer cells. Using a xenograft model of established human colon cancer, it was demonstrated that the SpeC-based TTS was able to control the growth and spread of large tumours in vivo. This required both TAA targeting by scFv5T4 and functional SAg activity. These studies lay the foundation for the development of streptococcal SAgs as 'next-generation' TTSs for cancer immunotherapy.